Insulated glass units (IGUs) generally comprise a pair of glass sheets, maintained in a spaced apart relationship to each other by a spacer assembly, and a sealing assembly which extends around the periphery of the inner facing surfaces of the glass sheets to define a sealed and insulating air space between the glass sheets. Typically, the spacer assembly is a hollow form which extends around the periphery of the inside facing surfaces of the glass sheets and which is filed with a water-absorbent material, such as a molecular sieve or another dehydration element, to keep the enclosed air space dry. The inner surfaces of the glass sheets are attached to the outer surface of the spacer assembly by means of a sealant or adhesive. Generally, the sealant or adhesive is also used to seal the edges of the insulated glass unit so as to establish a barrier which prevents moisture from penetrating into the interior annular space of the unit.
The sealant must have a combination of properties for satisfactory use. For example, the sealant must have a very low moisture vapor transmission (MVT) rate so that moisture is prevented from entering the dry annular space between the panes of glass. Moisture in such space tends to condense on the interior faces of the panes, creating visibility and aesthetic problems. If the sealant does not have a satisfactory MVT rate, the longevity of the insulated unit may be severely reduced. The sealant should have good elongation and flexibility so that it “yields” during contraction and expansion of the insulated glass structure, for example, to relieve stress on the glass caused by changes in temperature. The sealant desirably also forms an excellent bond with the glass which is not degraded over long periods of use when exposed to sunlight, moisture, and large temperature changes. Tensile adhesion strength is an important indicator of bond strength.
Two of the major types of sealants currently used in the insulated glass industry are: (A) thermoplastic one-part hot melt butyl type sealants, and (B) the chemically-curing thermoset sealant products generally from the generic families of polysulfide, polyurethane, and silicone. Hot melt butyl insulated glass sealants have been used with moderate success for a number of years in the insulated glass industry. However, there are significant shortcomings with this technology that have limited the application of hot melt butyl insulated glass sealants. Primarily, the hot melt butyl is a thermoplastic material, and not a thermoset material. Thermoplastic sealants are well known to soften when exposed to heat. Therefore, the insulated glass units sold in the marketplace which employ thermoplastic sealants are known to flow or deform, when placed under load, to relieve such stresses. This characteristic is exaggerated at high temperatures, which can occur in insulated glass units, especially those utilizing solar control glass. As a result, insulated glass units made with hot melt butyl sealants have difficulty passing stress and temperature tests common in industry, and are often limited for use in relatively small, light insulated glass units. Additionally, extreme care must be taken to support the insulated glass unit during handling, shipping and installation, resulting in additional costs. Furthermore, the hot melt sealants previously employed must be applied to the insulated glass units at temperatures exceeding 300° F. These high temperature requirements often present increased manufacturing costs, for example due to higher energy consumption and the need for specialized high-temperature equipment, as well as operational and safety challenges. Attempts to utilize lower temperature hot melts have been known to cause flow problems with the sealant.
The thermoset products which are currently used are generally two-component sealants which are mixed at the point of application at room temperature. The sealants then cure slowly by reaction with a supplied catalyst or through reaction with moisture. This slow cure requires that the insulated glass units be held in inventory from several hours to days waiting for the sealant to harden. Several single-component sealants are also available in the marketplace, such as those which include a partially cross-linked hot melt butyl rubber sealant. These single-component sealants, however, generally require treatment at elevated temperatures from about 325° F. to about 425° F. to crosslink the sealant. Other sealants employed in the art utilize urethane-curing chemistry, which is unsuitable for insulated glass industry because the carbon dioxide (CO2) generated in the process as bubbles can get trapped at the interface of the sealant and the glass which detrimentally affect the visibility and aesthetics of the insulated glass unit.
More recently, sealants based on polyurethane chemistry have been used for insulated glass units. These polyurethane-based sealants employ polyols, such as hydroxyl-terminated polybutadiene, to react with isocyanate to form a sealant. However, such sealants have environmental and safety concerns due to the utilization of isocyanates. As known to one having ordinary skill in the art, isocyanates are a family of highly reactive, low molecular weight chemicals. Isocyanates are powerful irritants to the mucous membranes of the eyes and gastrointestinal and respiratory tracts. Direct skin contact can also cause marked inflammation. Prolonged exposure can also sensitize workers, making them subject to severe asthma attacks or death if they are exposed again. Accordingly, compositions which have the beneficial properties of known insulated glass sealants, without the harmful safety concerns or detrimental by-products are highly desirable.